This invention applies to airplane propellers and horizontal-shaft wind turbine rotors that have variable pitch control of their individual blades, and particularly to the cases where such control is achieved by means of a passive balance of torques resulting from the application of springs and speed-sensitive and/or aerodynamic forces acting upon the pitch axis. (The terms “propeller” and “rotor” will be used interchangeably except as otherwise noted).
In the prior art, it has been customary to retain each individual blade relative to a central rotor or propeller hub by means of rolling-element thrust bearings to permit angular variation of the blade pitch. The blade-root hub often comprises a spindle or pitch shaft telescoped within and relatively movable angularly via two spaced radial bearings that define a pitch or feathering axis with respect to an outer housing or structure, where one of the spindle or housing is attached along the pitch axis to the individual blade and the other to the central rotational axis of the rotor. Within the configuration of the hub, between or outboard of the two radial bearings, means are provided for transfer of thrust loads between the spindle and the housing, Ball or roller thrust bearings may be heavily loaded by the centrifugal forces experienced by each blade due to its mass as it rotates rapidly about the central axis of the rotor.
Despite rolling-element bearings generally having a relatively low coefficient of friction, the high centrifugal thrust load may cause friction torque not only to be substantial compared to the torque available to overcome it, but it also has other undesirable characteristics. For instance, ball or roller bearings, particularly upon wear and lack of lubrication may develop a “lumpiness” or unevenness in their friction torque as they rotate through an angle of pitch, interrupting the smooth application of changes in control torque and resulting in uneven pitch angular response. This incremental change of torque is extreme, however, upon reversal of direction: friction torque of a rolling-element bearing abruptly reverses when the direction of feathering motion of the blades is reversed (as will be seen in the instant FIG. 1), making for a “jump” or step in pitch angle change rather than a smooth and accurate response to the controlling torque.
This unevenness of response may be particularly noticeable when the net torque upon the pitch axis is the result of a balance between torques applied by a calibrated spring and those developed as a function of rotational speed (RPMs) about the central axis and/or aerodynamic forces acting upon the blades The intent for such systems is that the system will be driven to a desired equilibrium pitch angle at which the net torque is near zero.
Passive RPM control is thus typically achieved through the action of a spring that manifests a torque tending to reduce blade pitch in opposition to a torque implemented through suitable linkages from centrifugal forces upon flyweights that urge the pitch of the blades to be increased when speed increases, thereby causing aerodynamic drag to slow the propeller down. This is the basis for passive constant-speed control mechanisms for aircraft propellers. Aerodynamic force acting upon the blades to produce feathering-axis torque is also involved in pitch control with the so-called Aeromatic propeller.
In both cases, accuracy and smoothness of control can be affected by friction torque changes of rolling-element thrust bearings: roughness or abrupt steps in rolling-element friction torque adding in to the net torque on a blade may then result in an equilibrium pitch angle that is offset from that achievable in the absence of such bearing friction, to the detriment of accuracy of control.
In an attempt to increase accuracy of control in such cases, the magnitude of the calibrated spring, RPM and aerodynamic torques might be intentionally increased relative to the undesirable rolling-element bearing torques, leading to increased bulk and weight of a design.